Re: Directional Mic - using only aural tubes

[EMAIL PROTECTED] (Mike Halloran) wrote in message news:<[EMAIL PROTECTED]>...
> A prototype of that last one should cost you less than $20 at Home Depot.
> 
> But hang on a sec.  Check it for function at home first, then check with your
> local materials experts before bringing it to the factory.
> 
> It sounds like you're working in a munitions factory.  In a place like that,
> I'd be hesitant to use something like the flexible clear vinyl tubing that
> first came to mind, not because it's conductive, but because it's so
> nonconductive that it can hold and carry a static charge from the factory where
> it was made to your factory.  Unrolling a package will sometimes cause the hair
> on your arm to stand up; probably not a good thing where you work.  
> 
> I think rigid, opaque (filled) pvc tubing and pipe may be conductive enough to
> bleed off static charge, but it's probably not _guaranteed_ to do so.  Again,
> check with your local experts.
> 
> -Mike-

What you're looking for is some kind of design for acoustical cavity
filters.  The phone company has always done pretty well transmitting
300 Hz to 3000 Hz for speech, so you need a low pass filter with
cutoff above 3k.  Maybe some research in how telephone companies used
to do it? Or duct and muffler silencing literature?  I know you do
that with impedance mismatch but don't remember how you went about
calculating the different tubing diameter sizes.